restructure: flatten workspace nesting, move devcontainer to root

- backend/workspaces/backend/ → backend/
- backend/workspaces/frontend/ → frontend/
- backend/.devcontainer/ + .vscode/ → repo root (where VS Code expects them)
- loose scripts/SQL files → scripts/
- replace nested git repo with single repo at project root
- update docker-compose.yml build context and devcontainer.json path
- add root .gitignore

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.devcontainer/devcontainer.json

@@ -1,7 +1,7 @@
 {
   "name": "Java Backend",
   "dockerComposeFile": [
-    "../../docker-compose.yml" 
+    "../docker-compose.yml"
   ],
   "service": "backend",
   "workspaceFolder": "/workspaces/${localWorkspaceFolderBasename}",